The use of microwave energy in both domestic and commercial establishments has become increasingly popular in the preparation of various food products. Where the food product is initially in a frozen state, thawing heating and/or cooking thereof can be readily expedited and at a low cost by utilizing such energy and at the same time enhancing the taste and nutritional value of the product. The time required to properly prepare the food product by utilizing microwave energy will depend upon a variety of factors, such as the product size and configuration; its density and consistency, and dielectric properties. Furthermore, in many instances in order to enhance its esthetic or visual appeal, it is necessary to brown or crisp the exterior of the food product or at least a portion thereof. Where the food product is a composite of various ingredients, it is sometimes necessary that certain of the ingredients requires enhanced heat during a given cooking or heating cycle within the microwave oven. To effect such heating variations, the receptacle or package in which the product is disposed during the cooking or heating cycle, may have certain areas thereof provided with means for reflecting the microwaves and thus shielding the adjacent portion of the food product therefrom and reducing the external and internal heat thereof. In other instances means can be provided at certain locations and areas of the receptacle or package wherein the microwaves are absorbed to a greater extend thereby generating surface areas of enhanced heat. Such variable heat requirements are particularly important where the receptacle or package segregates the various food products into contiguous compartments, such as occurs in various frozen TV dinners presently available on the market.
Heretofore difficulty has been experienced in economically producing a package or receptacle which meets the aforenoted heating requirements and is capable of withstanding extreme temperature variations such as occurs with microwavable frozen food products. Such prior packages frequently required various heat enhancing inserts which complicated the formation and setup of the package and caused the initial cost thereof to be inordinately high.
In other prior packages and receptacles of this general type, patches or layers of metallized film are laminated on the blanks of paperboard utilized in forming the package or receptacle components. Such metallized film is expensive; available from only a limited number of sources; requires expensive, specialized laminating equipment; and in some instances delamination of the film occurs resulting in ineffective temperature control in designated surface areas.
The enhancer or receptor metallized material often times increases the heat surface temperature of the substrate material, e.g., paperboard, on which it is deposited to a point where the substrate material becomes discolored or charred lessening its esthetics. Where the metallized material is applied to plastic containers, the latter may bow, curl or melt, when subjected to the generated microwave, causing contamination of the accommodated food product or leakage of the food product from the container onto the floor of the microwave oven creating a clean-up problem for the user.
Where conventional printing inks are applied directly on the exterior surface of the container, for graphic or informational purposes, such inks may be adversely affected by the increased surface temperatures of the container causing same to melt and/or be transferred to the microwave oven floor creating not only a clean-up problem, but in some cases, actual bonding of the container to the oven floor.
Frequently, prior disposable microwave oven food product packages and receptacles embodying enhancer features required a combination of diverse materials and non-conventional methods of combining such materials to form a usable and practical structure. For example, numerous prior containers of this general type, such as disclosed in U.S. Pat. No. 4,641,005, utilize a film of polyethylene terephthalate (PET) commonly in a thickness of 0.48 gauge which is then vacuum metallized with aluminum. This material must be tightly controlled as to the amount of metallization applied (often measured and referred to as optical density, light transmission or OHMS per square inch). This metallized film must then be laminated to a more durable substrate, such as paper or paperboard, to prevent said film from biaxially shrinking during cooking which will cause the prior controlled amount of aluminum particles deposited on the film to gather more closely together thereby increasing the materials's actual and prior controlled optical density to a non-controlled state thereby creating possible malfunctions of the receptive material itself and the quality of the cooked food in direct contact therewith.
An additional concern in this laminating process of the metallized PET film to paper or paperboard, is in finding an adhesive that will maintain its integrity during laminating, handling and microwave oven heating and cooking.
Traditionally, this form of microwave oven receptive packaging remains costly not only due to the amount of materials which must be combined in order to achieve the desired end result, but also due to the fact that diverse component suppliers and processors are often times involved such as a film supplier, a metallizer, a laminator, a paperstock supplier, and paperboard converter.